1. Field of the Invention
The present invention relates to a variable-focal-length lens and more particularly relates to a variable-focal-length lens wherein an electrooptic crystal is disposed in a slanted electric field and the lens action of the crystal is controlled by means of an electric signal.
2. Description of the Prior Art
In known optical systems of varifocal lenses, which are generally called zoom lenses, the change of the focal length is achieved by moving a selected lens group or groups along the optical axis thereby changing the distance between the lens groups. The lens system, therefore, needs to have moving mechanisms for the movement of the lens groups. However, these moving mechanisms have not satisfactorily ensured quick or high-speed changing of the focal length, and reduction in size and manufacturing cost.
As a solution for such problems, it has been proposed to make use of an electrooptic effect. In Japanese Patent Publication No. 615/1974, a variable-focal-length lens is disclosed wherein a slanted electric field is applied to an electrooptic crystal to control the lens action of the crystal by means of an electric signal. This type of variable-focal-length lens assures reduction in size and high-speed changing of the focal length.
FIGS. 1 and 2 show such prior art. In FIG. 1, designated by 1 is an electrooptic crystal, LiTaO.sub.3 ; and 2 to 5, are cylindrical electrodes to which an electric field is applied so that the electrodes 2 and 3 become positive and the electrodes 4 and 5 become negative. The electric field Ez in the direction of z-axis in FIG. 1 shows a particular distribution of electric field on y-axis wherein the electric field Ez is lower at and near the center of the crystal and becomes higher toward the edges of the crystal. Under these conditions, when light polarized in the direction of z-axis enters into the electrooptic crystal, the distribution of refractive index with respect to the direction of y-axis is such that the refractive index is maximum at the middle portion and gradually decreases toward the end portions of y-axis. Owing to this particular gradient of refractive index on y-axis, the incident light is concentrated to the middle of the crystal. In other words, this crystal acts as a cylindrical lens with a power in the direction of y-axis with respect to the light polarized in the direction of z-axis. The focal length of this cylindrical lens can be changed by changing the voltage applied between the cylindrical electrodes.
FIG. 2 shows a two-dimensional composite lens formed according to the above-mentioned concept.
Designated by 6 and 8 in FIG. 2 are cylindrical lenses each formed of an electrooptic crystal. The cylindrical lenses 6 and 8 are different from each other with respect to the direction of axis along which the refractive index varies. Denoted by 7 is a halfwave plate for rotating only the polarization plane through 90 degrees without changing the pattern.
The above described variable-focal-length lens according to the prior art has, however, some drawbacks.
First, it is difficult to provide cylindrical electrodes in an electrooptic crystal as shown in FIG. 1. This involves a substantial cost.
Second, it is by means possible to make a lens having a negative power, that is, so-called concave lens.
Third, when a two-dimensional composite lens as shown in FIG. 2 is to be made, it is absolutely necessary to use a half wavelength plate, which is quite disadvantageous for a compact structure of a two-dimensional lens. Furthermore, it leads to increase in cost.
As another approach, a one-dimensional variable-focal-length lens using an electrooptic crystal is disclosed in "Experiments on light deflectors and variable-focal-length lenses by applying slanted electric field on electrooptic crystal" by M. Sakaguchi (Proceedings of the 1975 National Meeting of Japanese Electronic Communication Society, 864, 1975). This lens system is shown in FIG. 3.
Designated by 11 in FIG. 3 is an electrooptic crystal, LiTaO.sub.3 ; and 12.sub.-n, . . . 12.sub.0, 12.sub.1, . . . 12.sub.n are slit-shaped electrodes. Denoted by 13 is a flat electrode, and by 14 is a lead wire. From the lead wire 14 there is applied to the crystal 11 an electric field approximated to a square distribution from middle to end, by the slit-shaped electrodes 12.sub.-n, . . . , 12.sub.0, 12.sub.1, . . . 12.sub.n. Thus, such refractive index distribution is realized which has a lens action to the light polarized in the direction of z-axis. By changing the electric field to be applied there is obtained a one-dimensional lens of variable-focal-length. In the example shown in FIG. 3, the electrodes can be formed in a simple manner employing a well-known metal electrode forming process such as spattering. It has another advantage that a concave lens can be obtained only by reversing the distribution of the electric field applied to the slit-shaped electrodes.
However, the example shown in FIG. 3 is limited to a one-dimensional lens only, and could not be applied to a variable-focal-length lens having two-dimensional lens action.